Our observation that protease inhibitors block tumor promotion has grown into a tool for chemo prevention of cancer. The work of the last two years demonstrates that diets containing soybeans delay the onset of two-state carcinogenesis using nitroquinoline oxide as the initiator and PMA as the promoter: delays and diminishes breast tumor response in Sprague-Dawley rats treated with X-rays and delays changes in the bladder epithelium caused by the bladder carcinogen FANFT in Fischer rats. We plan to confirm in this years work that, indeed, a protease inhibitor is responsible for the apparent prevention of cancer. We failed to get a precipitating antibody by injection of purified Kunitz soybean trypsin in food pads of rabbits. To confirm the appearance of soybean inhibitor trypsin, inhibitor (SBTI), in the tissues where tumors are observed, we will prepare radioactive soybean trypsin inhibitor by introducing the tritium or iodine into SBTI. We will test if smaller units of the inhibitors are still capable of inhibiting proteases. We plan to digest purified soybean trypsin inhibitors and soybean with pepsin at pH2 and isolate new inhibitors formed by affinity chromatography using trypsin and chymotrypsin Sephadex columns. Using radioactive SBTI as the source material, we hope to identify smaller units of the soybean trypsin inhibitor. We will lo k for such fragments in skin and breast tissue.